Cathode active material for lithium secondary battery and method of manufacturing the same

ABSTRACT

A cathode active material for a lithium secondary battery includes a lithium metal oxide particle, and an organic poly-phosphate or an organic poly-phosphonate formed on at least portion of a surface of the lithium metal oxide particle. Chemical stability of the lithium metal oxide particle may be improved and surface residues may be reduced by the organic poly-phosphate or the organic poly-phosphonate.

CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.16/692,895 filed on Nov. 22, 2019, which claims priority to KoreanPatent Applications No. 10-2018-0147722 filed on Nov. 26, 2018 in theKorean Intellectual Property Office (KIPO), the entire disclosure ofwhich is incorporated by reference herein.

BACKGROUND 1. Field

The present invention relates to a cathode active material for a lithiumsecondary battery and a method of manufacturing the same. Moreparticularly, the present invention relates to a lithium metaloxide-based cathode active material for a lithium secondary battery anda method of manufacturing the same.

2. Description of the Related Art

A secondary battery which can be charged and discharged repeatedly hasbeen widely employed as a power source of a mobile electronic devicesuch as a camcorder, a mobile phone, a laptop computer, etc., accordingto developments of information and display technologies. Recently, thesecondary battery or a battery pack including the same is beingdeveloped and applied as an eco-friendly power source of an electricautomobile such as a hybrid vehicle.

The secondary battery includes, e.g., a lithium secondary battery, anickel-cadmium battery, a nickel-hydrogen battery, etc. The lithiumsecondary battery is highlighted due to high operational voltage andenergy density per unit weight, a high charging rate, a compactdimension, etc.

For example, the lithium secondary battery may include an electrodeassembly including a cathode, an anode and a separation layer, and anelectrolyte immersing the electrode assembly. The lithium secondarybattery may further include an outer case having, e.g., a pouch shape.

A lithium metal oxide may be used as a cathode active material of thelithium secondary battery, and a nickel-based lithium metal oxide may beused as the lithium metal oxide.

As an application of the lithium secondary battery has been expanded,demands of more improved life-span, capacity and operation stability areincreased. In the lithium metal oxide used as the cathode activematerial, non-uniformity of a chemical structure due to lithiumprecipitation may be caused, and the lithium secondary battery havingdesired capacity and life-span may not be obtained. Further, a structureof the lithium metal oxide may be transformed or damaged when chargingand discharging operations are repeated to degrade life-span stabilityand capacity retention.

For example, Korean Published Patent Application No. 10-0821523discloses a method of removing lithium salt impurities by washing alithium complex metal oxide with water. However, the impurities may notbe sufficiently removed by the method, and surface damages of cathodeactive material particles may be caused during the washing process.

SUMMARY

According to an aspect of the present invention, there is provided acathode active material for a lithium secondary battery having improvedoperational stability and electrical property and a method ofmanufacturing the same.

According to an aspect of the present invention, there is provided alithium secondary battery having improved operational stability andelectrical property.

According to exemplary embodiments of the present invention, a cathodeactive material for a lithium secondary battery includes a lithium metaloxide particle, and an organic poly-phosphate or an organicpoly-phosphonate formed on at least portion of a surface of the lithiummetal oxide particle.

In some embodiments, the organic poly-phosphate is derived from anorganic poly-phosphate compound represented by Structural Formula 1below or an organic poly-phosphate salt thereof, and the organicpoly-phosphonate is derived from an organic poly-phosphonate compoundrepresented by Structural Formula 2 below or an organic poly-phosphonatesalt thereof.

In the Structural Formulae 1 and 2 above, m may be an integer from 2 to20, R₁ may represent a C1-C10 hydrocarbon group capable of substitutedwith a substituent group, and the substituent group may include halogen,a cyano group, a hydroxyl group, a phosphoric acid group, a carboxylicgroup or a salt thereof.

In some embodiments, the hydrocarbon group included in R₁ may besubstituted or connected by at least one selected from a groupconsisting of a carbon-carbon double bond, —O—, —S—, —CO—, —CO——, —SO—,—CO—O—, —O—CO—O—, —S—CO—, —S—CO—O—, —CO—NH—, —NH—CO—O—, —NR′—,

—R′OH—, —S—S— and —SO₂—, and R′ is hydrogen or a C1-C8 alkyl group.

In some embodiments, the organic poly-phosphate compound or the organicpoly-phosphonate compound may include at least one of compoundsrepresented by Chemical Formulae 1 to 3 below.

In some embodiments, the organic poly-phosphate salt or the organicpoly-phosphonate salt includes a compound represented by ChemicalFormula 4 below.

In the Chemical Formula 4 above, n may be an integer from 2 to 10.

In some embodiments, the lithium metal oxide particle includes anickel-based lithium oxide represented by General Formula 1 below.

Li_(x)Ni_(y)M_(1-y)O₂  [General Formula 1]

In the General Formula 1 above, 0.95≤x≤1.08, y≥0.5, and M may be atleast one element selected from a group consisting of Co, Mn, Al, Zr,Ti, B, Mg and Ba.

In some embodiments, in the General Formula 1, 0.8≤y≤0.93.

In some embodiments, in the General Formula I, M may include Co and Mn.

In some embodiments, the lithium metal oxide particle may include adoping or a coating which contains at least one of Al, Zr or Ti.

In some embodiments, the organic poly-phosphate or the organicpoly-phosphonate may form a coating layer, a ligand bond or a complexbond on the surface of the lithium metal oxide particle.

In some embodiments, the lithium metal oxide particle has a layerstructure, and a grain boundary at a surface portion of the lithiummetal oxide particle may be coated by the organic poly-phosphate or theorganic pol phosphonate.

According to exemplary embodiments of the present invention, in a methodof preparing a cathode active material for a lithium secondary battery,a lithium metal oxide particle is prepared. The lithium metal oxideparticle is cleaned using a washing solution that includes an organicpoly-phosphate compound or an organic poly-phosphonate compound.

In some embodiments, an adding amount of the organic poly-phosphatecompound or the organic poly-phosphonate compound may be in a range from0.1 weight percent to 2 weight percent based on a total weight of thelithium metal oxide particle.

In some embodiments, an adding amount of the organic poly-phosphatecompound or the organic poly-phosphonate compound may be in a range from0.2 weight percent to 1 weight percent based on a total weight of thelithium metal oxide particle.

In some embodiments, before cleaning the lithium metal oxide particle,the lithium metal oxide particle may be mixed and annealed with at leastone of Al₂O, ZrO₂ or TiO₂.

According to exemplary embodiments of the present invention, a lithiumsecondary battery includes a cathode including a lithium metal oxideparticle and an organic poly-phosphate or an organic poly-phosphonateformed on at least portion of a surface of the lithium metal oxideparticle, an anode and a separation layer interposed between the cathodeand the anode.

According to exemplary embodiments of the present invention, an organicpoly-phosphate or an organic poly-phosphonate may be formed on a surfaceof a lithium metal oxide particle so that a metal ion complex may beformed when a layer structure of the lithium metal oxide particle isdamaged. Thus, structural or crystalline stability may be enhancedcompared to when an inorganic phosphoric acid or an inorganic phosphoricacid salt may be used, and stability and capacity/power output of acathode active material may be also improved.

In exemplary embodiments, the organic poly-phosphate or the organicpoly-phosphonate may be introduced by a cleaning process utilizing awashing solution. Accordingly, impurities such as lithium saltprecipitates remaining on the surface of the lithium metal oxideparticle may be removed while forming the organic poly-phosphate or theorganic poly-phosphonate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing capacity retentions of battery cells accordingto Example 3-3, Comparative Example 4, Comparative Example 5 andComparative Example 7 during repeated charging/discharging operations.

FIG. 2 is a TOF-SIMS analysis image of a surface of a lithium metaloxide particle according to Example 3-3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

<Cathode Active Material and Method of Manufacturing the Same>

A cathode active material for a lithium secondary battery (hereinafter,abbreviated as a cathode active material) according to exemplaryembodiments may include a lithium metal oxide particle and an organicpoly-phosphate coating or an organic poly-phosphonate coating(hereinafter, also abbreviated as an organic poly-phosphate/phosphonatecoating) formed on a surface of the lithium metal oxide panicle.

The term “lithium metal oxide” used herein indicates a complex oxideincluding lithium and at least one metal except for lithium. Inexemplary embodiments, the lithium metal oxide may include anickel-based lithium oxide.

For example, the nickel-based lithium oxide may be represented byGeneral Formula 1 below.

Li_(x)Ni_(y)M_(1-y)O₂  [General Formula 1]

In the General Formula 1 above, 0.95≤x≤1.08, y≥0.5, and M may be atleast one element selected from Co, Mn, Al, Zr, Ti, B, Mg or Ba.

For example, in the lithium metal oxide of General Formula 1, nickel(Ni) may be an element related to a capacity of a lithium secondarybattery. For example, as an amount of nickel becomes greater, a capacityand a power output of the lithium secondary battery may be improved.

In an embodiment, 0.8≤y≤0.93 in General Formula 1 so that a cathodeactive material providing high capacity and power output may be easilyobtained.

M may include cobalt (Co) and manganese (Mn) so that electricalconductivity and chemical stability may be added to a high-Ni compound.For example, cobalt (Co) may be an element related to a conductivity anda resistance of the lithium secondary battery. In an embodiment, M mayinclude manganese (Mn) and Mn may be an element related to mechanicaland electrical stability of the lithium secondary battery.

Accordingly, the lithium metal oxide particle may include anickel-cobalt-manganese based compound, and the cathode active materialhaving improved capacity, power output, low resistance and life-span maybe provided.

In some embodiments, the lithium metal oxide particle may furtherinclude a doping element or a coating element in addition to Ni, Co andMn. For example, the doping or coating element may include Al, Zr and/orTi, and may preferably include Al, Zr and Ti.

In the nickel-cobalt-manganese based lithium oxide, an amount of thedoping element may be in a range from about 0.1 mol % to about 1 mol %based on a total mole of Ni, Co, Mn and the doping element (e.g., Al, Zrand/or Ti), preferably in a range from about 0.5 mol % to about 1 mol %.Within the above range, chemical and structural stability of the lithiummetal oxide particle may be enhanced without excessively degrading anactivity of the cathode active material.

The coating element may be derived from a coating metal oxide such asAl₂O₃, ZrO₂ and/or TiO₂. An adding amount of the coating metal oxide fora formation of the coating may be in a range from about 0.5 wt % toabout 1 wt % based on a total weight of the nickel-cobalt-manganesebased lithium oxide.

In some embodiments, the lithium metal oxide particle may have a layerstructure. For example, primary particles of the lithium metal oxide maybe agglomerated into the layer structure to form the lithium metal oxideparticle as the cathode active material. A mobility of lithium ionsgenerated from the cathode active material may be facilitated throughthe particle structure.

In some embodiments, the organic poly-phosphate/phosphonate coating maybe formed on the surface of the lithium metal oxide particle.

The term “organic poly-phosphate/phosphonate coating” used herein mayinclude a coating layer formed on a substantially entire surface of thelithium metal oxide particle, and a coating layer or an island-shapelayer formed on a portion of the surface of the lithium metal oxideparticle.

In some embodiments, the organic poly-phosphate/phosphonate coating mayinclude an organic ligand bond attached on the surface of the lithiummetal oxide particle, or an organic-metal complex combined with a metalion exposed on the surface of the lithium metal oxide particle.

In some embodiments, a grain boundary at a surface portion of thelithium metal oxide particle may be coated by the organicpoly-phosphate/phosphonate coating.

In exemplary embodiments, the organic poly-phosphate coating may bederived from an organic poly-phosphate compound or an organicpoly-phosphate salt. The organic poly-phosphonate coating may be derivedfrom an organic poly-phosphonate compound or an organic poly-phosphonatesalt.

In the present specification, the organic poly-phosphate compound mayalso indicate the organic poly-phosphate salt, and the organicpoly-phosphonate compound may also indicate the organic poly-phosphonatesalt.

In some embodiments, the organic poly-phosphate compound may include acompound represented by Structural Formula 1 below.

In some embodiments, the organic poly-phosphonate compound may include acompound represented by Structural Formula 2 below.

In the structural Formulae 1 and 2 above, m may be an integer from 2 to20, and R₁ may represent a C1-C10 hydrocarbon group capable ofsubstituted with a substituent group. The substituent group may includehalogen, a cyano group, a hydroxyl group, a phosphoric acid group, acarboxylic group or a salt thereof.

The term “hydrocarbon group” used herein may include a cyclic aliphaticgroup, a linear aliphatic group, an aromatic group or a combinationthereof. For example, the hydrocarbon group may include an alkyl group,an alcohol group, an alkoxy group, an aryl group (e.g., C6-C10), an arylalkoxy group (e.g., C6-C10), a cyclo alkyl group (e.g., C3-C10) or amulti-cyclic group (e.g., C5-C10).

In some embodiments, the hydrocarbon group included in R₁ may besubstituted or connected by at least one selected from a groupconsisting of a carbon-carbon double bond, —O—, —S—, —CO—, —OCO—, —SO—,—O—CO—O—, —S—CO—, —S—CO—O—, —CO—NH—, —NH—CO—O—, —NR′—, —R′OH—,

—S—S— and —SO₂—, R′ may be hydrogen or a C1-C8 alkyl group.

In some embodiments, the organic poly-phosphate compound or the organicpoly-phosphonate compound may include at least one of compoundsrepresented by Chemical Formula 1 (inositol hexaphosphate or phyticacid), Chemical Formula 2 (etidronic acid) and Chemical Formula 3(nitrilotris(methylene)triphosphonic acid).

In some embodiments, the organic poly-phosphate salt or the organicpoly-phosphonate salt may include a compound represented by ChemicalFormula 4 below.

In the Chemical Formula 4 above, n may be an integer from 2 to 10.

The above mentioned organic poly-phosphate compound or the organicpoly-phosphonate compound (hereninafter, abbreviated as an organicpoly-phosphate/phosphonate compound) may form a ligand bond or a complexbond on the surface of the lithium metal oxide particle so that a largenumber of stable bonds with metal ions exposed on the surface of thelithium metal oxide particle may be formed. Thus, the surface of thelithium metal oxide particle may be protected from, e.g., a sidereaction with an electrolyte effectively compared to when phosphoricacid or inorganic phosphate may be added.

For example, when phytic acid of Chemical Formula 1 may be used as theorganic poly-phosphate/phosphonate compound, one molecule of phytic acidmay react simultaneously with mono-valent ion such as a lithium ion,di-valent ion such as a nickel ion and tri-valent ion such as a cobaltion or an aluminum ion to form bonds as shown in Structural Formula 3.

Accordingly, even when a layer structure of the lithium metal oxideparticle may be transformed or damaged due to repeatedcharging/discharging operations, multi-bonds or multi-complexes may beformed with exposed or discharged metal ions by the organicpoly-phosphate/phosphonate compound so that by-products caused by aside-reaction of the metal ions may be prevented or reduced.

Further, if the lithium metal oxide particle includes the additionaldoping or coating, the organic poly-phosphate/phosphonate compound maybe combined with the doping or coating to further improve chemicalstability of the cathode active material.

Hereinafter, a method of preparing the cathode active material will bedescribed in more detail.

In exemplary embodiments, a lithium precursor and a nickel precursor maybe reacted with each other to form the lithium metal oxide particle. Thelithium precursor and the nickel precursor may include an oxide or ahydroxide of lithium and nickel, respectively. For example, the lithiumprecursor and the nickel precursor may be reacted in a solution by aprecipitation reaction such as a co-precipitation to form a preliminarylithium metal oxide.

In some embodiments, another metal precursor (e.g., a cobalt precursor,a manganese precursor, etc.) in addition to the lithium precursor andthe nickel precursor may be also reacted. In some embodiments, anickel-cobalt-manganese precursor e.g., a Ni—Co—Mn hydroxide) may beused together with the lithium precursor.

Another metal precursor may include a precursor of Al, Zr and/or Ti inaddition to the cobalt precursor and the manganese precursor inconsideration of forming the doping.

In some embodiments, after preparing the lithium metal oxide, a firingprocess fora calcination process) may be further performed. For example,the firing process may be performed at a temperature in a range fromabout 600° C. to about 1,000° C. The layer structure of the lithiummetal oxide particle may be stabilized by the firing process and thedoping element may be fixed.

In some embodiments, the lithium metal oxide particle may be mixed witha metal oxide for farming a coating such as Al₂O₃, ZrO₂ and/or TiO₂, andthen an additional annealing process may be further performed to form acoating.

In exemplary embodiments, the lithium metal oxide particle may be washedor cleaned using a washing solution that contains the organicpoly-phosphate/phosphonate compound.

Non-reacted precursors may be remained or precipitated on a surface ofthe lithium metal oxide particle synthesized by the precursor reactionas described above.

Further, impurities and solution molecules may be remained on thelithium metal oxide particle during the synthesis.

In some embodiments, an excess amount of the lithium precursor may beused for a production efficiency of the lithium metal oxide particle anda synthesis stability. In this case, lithium salt impurities including,e.g., lithium hydroxide (LiOH) and lithium carbonate (Li₂CO₃) may beremained on the surface of the lithium metal oxide particle.

The lithium salt impurities may be captured and removed by the organicpoly-phosphate/phosphonate compound included in the washing solution. Ina comparative example, when water is used in the washing solution, thelithium salt impurities may be also removed. However, an oxidation ofthe surface of the lithium metal oxide particle and a side reaction bywater may occur to cause damages of the layer structure in the cathodeactive material.

However, according to exemplary embodiments, the washing process may beperformed using the washing solution that includes the organic compoundso that the lithium salt impurities may be effectively removed through amulti-valent structure of the organic poly-phosphate/phosphonatecompound while preventing the oxidation and the layer structure damagesof the particle surface.

Additionally, the organic poly-phosphate/phosphonate coating may beformed as described above during the washing process so that removal ofthe impurities and. passivation of the particle surface may besimultaneously implemented.

In some embodiments, an amount of the organic poly-phosphate/phosphonatecompound may be in a range from about 0.1 wt % to about 2 wt % based ona total weight of the lithium metal oxide particle. Within this range,sufficient coating and passivation may be achieved without anexcessively degrading activity of the metal included in the cathodeactive material.

Preferably, the amount of the organic poly-phosphate/phosphonatecompound may be in a range from about 0.2 wt % to about 1 wt % based onthe total weight of the lithium metal oxide particle.

In some embodiments, the washing solution may be prepared by dissolvingthe organic poly-phosphate/phosphonate compound in water. In anembodiment, an organic solvent such as an alcohol-based solvent may beused in the washing solution.

In some embodiments, a drying process may be further performed after thewashing process. The organic poly-phosphate/phosphonate coating may befixed or stabilized on the particle surface by the drying process.

≤Lithium Secondary Battery>

According to exemplary embodiments, a lithium secondary battery mayinclude a cathode including the lithium metal oxide particle on whichthe organic poly-phosphate/phosphonate coating may be formed, an anodeand a separation layer.

The cathode may include a cathode active material layer formed bycoating a cathode active material that may include the lithium metaloxide particle on a cathode current collector.

For example, the lithium metal oxide particle may be mixed and stirredtogether with a binder, a conductive agent and/or a dispersive agent ina solvent to form a slurry. The slurry may be coated on the cathodecurrent collector, and pressed and dried to obtain the cathode.

The cathode current collector may include stainless-steel, nickel,aluminum, titanium, copper or an alloy thereof. Preferably, aluminum oran alloy thereof may be used.

The binder may include an organic based binder such as a polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinylidenefluoride (PVDF), polyacrylonitrile,polymethylmethacrylate, etc., or an aqueous based binder such asstyrene-butadiene rubber (SBR) that may be used with a thickener such ascarboxymethyl cellulose (CMC).

For example, a PVDF-based binder may be used as a cathode binder. Inthis case, an amount of the binder for forming the cathode activematerial layer may be reduced, and an amount of the cathode activematerial may be relatively increased. Thus, capacity and power output ofthe lithium secondary battery may be further improved.

The conductive agent may be added to facilitate an electron mobilitybetween the active material particles. For example, the conductiveadditive may include a carbon-based material such as graphite, carbonblack, graphene, carbon nanotube, etc., and/or a metal-based materialsuch as tin, tin oxide, titanium oxide, a perovskite material such asLaSrCoO₃ or LaSrMnO₃.

The anode may include an anode current collector and an anode activematerial layer formed by coating an anode active material on the anodecurrent collector.

The anode active material may include a material that may be capable ofadsorbing and ejecting lithium ions. For example, a carbon-basedmaterial such as a crystalline carbon, an amorphous carbon, a carboncomplex or a carbon fiber, a lithium alloy, silicon, tin, etc., may beused. The amorphous carbon may include a hard. carbon, cokes, amesocarbon microbead (MCMB) calcinated at a temperature of 1,500° C. orless, a mesophase pitch-based carbon fiber (MPCF), etc. The crystallinecarbon may include a graphite-based material such as natural graphite,graphitized cokes, graphitized MCMB, graphitized MPCF, etc. The lithiumalloy may further include aluminum, zinc, bismuth, cadmium, antimony,silicon, lead, tin, gallium, or indium.

The anode current collector may include gold, stainless-steel, nickel,aluminum, titanium, copper or an alloy thereof, preferably, may includecopper or a copper alloy.

In some embodiments, the anode active material may be mixed and stirredtogether with a binder, a conductive agent and/or a dispersive agent ina solvent to form a slurry. The slurry may be coated on the anodecurrent collector, and pressed and dried to obtain the anode.

The binder and the conductive agent substantially the same as or similarto those as mentioned above may be used. In some embodiments, the binderfor the anode may include an aqueous binder such as such asstyrene-butadiene rubber (SBR) that may be used with a thickener such ascarboxymethyl cellulose (CMC) so that compatibility with thecarbon-based active material may be improved.

The separation layer may be interposed between the cathode and theanode. The separation layer may include a porous polymer film preparedfrom, e.g., a polyolefin-based polymer such as an ethylene homopolymer,a propylene homopolymer, an ethylene/butene copolymer, anethylene/hexene copolymer, an ethylene/methacrylate copolymer, or thelike. The separation layer may be also formed from a non-woven fabricincluding a glass fiber with a high melting point, a polyethyleneterephthalate fiber, or the like.

In exemplary embodiments, an electrode cell may be defined by thecathode, the anode and the separation layer, and a plurality of theelectrode cells may be stacked to form an electrode assembly having,e.g., a jelly roll shape. For example, the electrode assembly may beformed by winding, laminating or folding of the separation layer.

The electrode assembly may be accommodated in an outer case togetherwith an electrolyte to form the lithium secondary battery. In exampleembodiments, the electrolyte may include a non-aqueous electrolytesolution.

The non-aqueous electrolyte solution may include a lithium salt and anorganic solvent. The lithium salt may be represented by Li⁺X⁻, and ananion of the lithium salt X⁻ may include, e.g., F⁻, Cl⁻, Br⁻, NO₃ ⁻,N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻,(CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻,CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻,CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, (CF₃CF₂SO₂)₂N⁻, etc.

The organic solvent may include propylene carbonate (PC), ethylenecarbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC),ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate,dimethyl sulfoxide, acetonitrile, dimethoxy ethane, diethoxy ethane,vinviene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite,tetrahydrofuran, etc. These may be used alone or in a combinationthereof.

An electrode tab may be formed from each of the cathode currentcollector and the anode current collector to extend to one end of theouter case. The electrode tabs may be welded together with the one endof the outer case to form an electrode lead exposed at an outside of theouter case.

The lithium secondary battery may be fabricated into a cylindrical shapeusing a can, a prismatic shape, a pouch shape, a coin shape, etc.

According to exemplary embodiments, chemical stability of the cathodeactive material may be enhanced by the organicpoly-phosphate/phosphonate coating so that life-span and long-termstability of the lithium secondary battery may be improved whilesuppressing a reduction of a capacity and an average voltage.

:Hereinafter, preferred embodiments are proposed to more concretelydescribe the present invention. However, the following examples are onlygiven for illustrating the present invention and those skilled in therelated art will obviously understand that various alterations andmodifications are possible within the scope and spirit of the presentinvention. Such alterations and modifications are duly included in theappended claims.

Examples and Comparative Examples

In Examples and Comparative Examples, lithium metal oxide particleshaving compositions as shown in Table 1 below(Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ or Li[Ni_(0.8)Co_(0.1)Mn_(0.1)]O₂were prepared.

Specifically, a lithium hydroxide as a lithium precursor and anickel-cobalt-manganese hydroxide as a complex metal precursor having amolar ratio corresponding to that shown in Table 1 were uniformly mixedby a molar ratio of 1.05:1 to form a mixture. The mixture was input in afiring chamber, oxygen was provided at a flow rate of 10 mL/min whileheating to a temperature between 700° C. and 800° C. at a rate of 2°C./min, and the mixture was maintained in the firing chamber for 10hours at the temperature. After the firing, the mixture was cooled toroom temperature, and then grinded and distributed to obtain the lithiummetal oxide particle as a cathode active material.

Examples

Organic poly-phosphate/phosphonate compounds as listed in Table 1 wereinput 100 mL of pure water having a resistivity of 20 MΩ cm or less toprepare a washing solution. The lithium metal oxide particles were inputin the washing solution, stirred for 10 minutes and filtrated. Thefiltrated lithium metal oxide particles were vacuum-dried at atemperature of 250° C. for 12 hours to obtain desired lithium metaloxides,

Comparative Examples

In Comparative Examples 1 to 4, processes the same as those of Exampleswere performed except that pure water having a resistivity of 20 MΩ cmor less was only used as a washing solution.

In Comparative Examples 5 and 6, processes the same as those of Exampleswere performed except that phosphoric acid or an inorganic phosphoricacid salt was used instead of the organic poly-phosphate/phosphonatecompounds.

In Comparative Example 7, processes the same as those of Examples wereperformed except that ammonium poly-phosphate devoid of carbon was usedinstead of the organic poly-phosphate/phosphonate compounds.

TABLE 1 Dopant (M) amount Lithium Metal M/(Ni + Co + Mn + M) CoatingAmount (wt %) Added Oxide Particle Al Zr Ti Al₂O₃ ZrO₂ TiO₂ Compound wt% Example 1-1 Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ Inositol 0.5hexaphosphate Example 1-2 Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ etidronicacid 0.5 Example 1-3 Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂Nitrilotris(methylene) 0.5 triphosphonic acid Example 1-4Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ Trimethylamine 0.5 triphosphateExample 2-1 Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ Inositol 2.0 hexaphosphateExample 2-2 Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ Inositol 1.0 hexaphosphateExample 2-3 Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ Inositol 0.2 hexaphosphateExample 2-4 LI[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ Inositol 0.1 hexaphosphateExample 3-1 Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ 0.3 0.2 0.2 Inositol 0.5hexaphosphate Example 3-2 Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ 0.3 0.2 0.2Nitrilotris(methylene) 0.5 triphosphonic acid Example 3-3Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ 0.5 0.06 0.2 Inositol 0.5hexaphosphate Example 3-4 Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ 0.5 0.06 0.2Nitrilotris(methylene) 0.5 triphosphonic acid Example 4-1Li[Ni_(0.8)Co_(0.1)Mn_(0.1)]O₂ Inositol 0.5 hexaphosphate Example 4-2Li[Ni_(0.8)Co_(0.1)Mn_(0.1)]O₂ Nitrilotris(methylene) 0.5 triphosphonicacid Example 4-3 Li[Ni_(0.8)Co_(0.1)Mn_(0.1)]O₂ 0.3 0.2 0.2 Inositol 0.5hexaphosphate Example 4-4 Li[Ni_(0.8)Co_(0.1)Mn_(0.1)]O₂ 0.3 0.2 0.2Nitrilotris(methylene) 0.5 triphosphonic acid Example 4-5Li[Ni_(0.8)Co_(0.1)Mn_(0.1)]O₂ 0.5 0.06 0.2 Inositol 0.5 hexaphosphateExample 4-6 Li[Ni_(0.8)Co_(0.1)Mn_(0.1)]O₂ 0.5 0.06 0.2Nitrilotris(methylene) 0.5 triphosphonic acid ComparativeLi[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ none Example 1 ComparativeLi[Ni_(0.8)Co_(0.1)Mn_(0.1)]O₂ none Example 2 ComparativeLi[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ 0.3 0.2 0.2 none Example 3 ComparativeLi[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ 0.5 0.06 0.2 none Example 4Comparative Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ 0.5 0.06 0.2 H₃PO₄ 0.5Example 5 Comparative Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ 0.5 0.06 0.2Li₃PO₄ 0.5 Example 6 Comparative Li[Ni_(0.88)Co_(0.09)Mn_(0.03)]O₂ 0.50.06 0.2 ammonium 0.5 Example 7 polyphosphate

Experimental Example

(1) Measurement of Remaining Lithium Salt Impurities

5.0 g of each lithium metal oxide particle according to Examples andComparative Examples was quantified in a 250 mL flask with 100 g ofdeionized water, and then a magnetic bar was put and stirred for 10minutes at a rate of 4 rpm. The mixture was filtered using a pressurereducing flask and 50 g of the mixture was collected. The collectedmixture was automatically titrated with 0.1N HCl in an auto titrator tomeasure amounts of LiOH and Li₂CO₃ as shown in Table 2 below.

(2) Evaluation of Battery Properties

2-1) Fabrication of Secondary Battery Cell

Each lithium metal oxide particles of Examples and Comparative Examples,carbon black as a conductive additive and PVDF as a binder were mixed bya weight ratio of 92:5:3 to form a slurry. The slurry was uniformlycoated on an aluminum foil having a thickness of 15 μm and vacuum-driedat 130° C. to form a cathode for a lithium secondary battery. Anelectrode assembly was formed using the cathode, a lithium foil as acounter electrode, a porous polyethylene layer (thickness: 21 μm) as aseparator. A battery cell having a coin half cell shape was fabricatedby a commonly known process using the electrode assembly and anelectrolyte solution in which 1.0 M of LiPF6 vas dissolved in a solutionincluding ethylene carbonate and ethyl methyl carbonate by a volumeratio of 3:7.

2-2) Measurement of Initial Charging Discharging Capacity

One cycle of a charging (CC/CV 0.1C 4.3V 0.05CA CUT-OFF) and adischarging (CC 0.1C 3.0V CUT-OFF) was performed to the battery cells ofExamples and Comparative Examples, and initial charging and dischargingcapacities were measured (CC: constant current, CV: Constant voltage)

2-3) Measurement of Initial Efficiency

The initial discharging capacity measured in the above 2-2) wasdenominated by an initial charging capacity to measure an initialefficiency as a percentage value.

2-4) Measurement of Capacity Retention

300 cycles of a charging (CC/CV 0.5C 4.3V 0.05CA CUT-OFF) and adischarging (CC 1.0C 3.0V CUT-OFF) were repeated using the battery cellsof Examples and Comparative Examples. A discharging capacity at the300th cycle was denominated by the discharging capacity at the firstcycle to measure a capacity retention ratio as a percentage value.

The results are shown in Table 2 below.

TABLE 2 Initial Initial Charging Discharging Initial Capacity LiOHLi2CO3 Capacity Capacity Effciency Retention wt % wt % mAh/g mAh/g % %Example 1-1 0.211 0.401 240 214 89 65 Example 1-2 0.232 0.374 242 216 8961 Example 1-3 0.200 0.412 243 213 88 63 Example 1-4 0.251 0.392 240 21088 58 Example 2-1 0.233 0.444 230 211 92 63 Example 2-2 0.201 0.399 239214 90 65 Example 2-3 0.222 0.309 238 214 90 63 Example 2-4 0.218 0.281238 214 90 61 Example 3-1 0.205 0.390 236 212 90 78 Example 3-2 0.2220.379 238 211 89 73 Example 3-3 0.197 0.382 238 215 90 80 Example 3-40.231 0.309 237 210 89 79 Example 4-1 0.123 0.384 226 199 88 73 Example4-2 0.189 0.355 228 198 87 70 Example 4-3 0.175 0.361 220 200 91 82Example 4-4 0.19 0.400 226 201 89 80 Example 4-5 0.181 0.349 222 202 9182 Example 4-6 0.209 0.412 224 202 90 78 Comparative 0.121 0.192 243 21187 28 Example 1 Comparative 0.111 0.198 228 201 88 33 Example 2Comparative 0.183 0.194 240 208 87 43 Example 3 Comparative 0.211 0.256240 211 87 45 Example 4 Comparative 0.071 0.638 247 201 81 30 Example 5Comparative 0.301 0.987 245 215 88 62 Example 6 Comparative 0.089 0.591236 205 87 31 Example 7

Referring to Table 2 above, when the washing process using the organicpoly-phosphate/phosphonate compound was performed, an amount of lithiumimpurities was decreased and improved charging/discharging efficiencyand capacity retention were obtained.

In Comparative Examples 1 to 4 in which water was only used in thewashing process, the lithium impurities were effectively removed, butefficiency and capacity retention were degraded due to an oxidation of aparticle structure.

FIG. 1 is a graph showing discharge capacity retentions of battery cellsaccording to Example 3-3 Comparative Example 4, Comparative Example 5and Comparative Example 7 during repeated charging/dischargingoperations.

Referring to FIG. 1, remarkably improved capacity retention was obtainedfrom Example 3-3 in which the washing process using the organicpoly-phosphate/phosphonate compound was performed compared to those fromComparative Examples.

(3) Surface Analysis

A surface of the lithium metal oxide particle prepared by Example 3-3 inwhich the washing process using the organic poly-phosphate/phosphonatecompound was performed was analyzed by a time of flight secondary ionmass spectroscopy (TOF-SIMS) method. Specifically, a Bi³+ ion gun wasused by a TOP-SIMS V (ION-TOF GmbH, Germany) apparatus equipped with aBi ion gun, and a analyzing area was 50×50 μm².

FIG. 2 is a TOF-SIMS analysis image of a surface of a lithium metaloxide particle according to Example 3-3.

As shown in FIG. 2, ion fragment peaks generated from the organicpoly-phosphate compound such as CxHyPO3+ and PO+ peaks uniformlydistributed on the particle surface of Example 3-3 were detected.

What is claimed is:
 1. A cathode active material for a lithium secondarybattery, comprising: a lithium metal oxide particle; and. an organicpoly-phosphate forming a coating layer on at least a portion of thelithium metal oxide particle or an organic poly-phosphonate forming acoating layer on at least a portion of the lithium metal oxide particle,wherein an organic ligand bond or an organic metal complex bond isformed between the lithium metal oxide particle and the coating layer,wherein, in a surface of the lithium metal oxide particle, ion fragmentpeaks generated from the organic poly-phosphate or the organicpoly-phosphonate is detected by a Time of Flight Secondary Ion MassSpectrometry (TOF-SIMS) analysis.
 2. The cathode active material for alithium secondary battery according to claim 1, wherein the ion fragmentpeak includes C₃H₇PO₃+.
 3. The cathode active material for a lithiumsecondary battery according to claim 1, wherein the organicpoly-phosphate is derived from an organic poly-phosphate compoundrepresented by Structural Formula 1 below or an organic poly-phosphatesalt thereof, and the organic poly-phosphonate is derived from anorganic poly-phosphonate compound represented by Structural Formula 2below or an organic poly-phosphonate salt thereof:

wherein, in the Structural Formulae 1 and 2 above, in is an integer from2 to 20, R₁ represents a C₁-C₁₀ hydrocarbon group substituted with asubstituent group or an unsubstituted C₁-C₁₀ hydrocarbon group, and thesubstituent group includes halogen, a cyano group, a hydroxyl group, aphosphoric acid group, a carboxylic group or a salt thereof.
 4. Thecathode active material for a lithium secondary battery according toclaim 3, wherein the hydrocarbon group included in R₁ is substituted orconnected by at least one selected from a group consisting of acarbon-carbon double bond, —O—, —S—, —CO—, —OCO—, —SO—, —CO—O—,—O—CO—O—, —S—CO—, —S—CO—O—, —CO—NH—, —NH—CO—O—, NR′—,

—R′OH, —S—S—, and —SO₂— and R′ is hydrogen or a C₁-C₈ alkyl group. 5.The cathode active material for a lithium secondary battery according toclaim 3, wherein the organic poly-phosphate compound or the organicpoly-phosphonate compound includes at least one of compounds representedby Chemical Formulae 1 to 3 below:


6. The cathode active material for a lithium secondary battery accordingto claim 3, wherein the organic poly-phosphate salt or the organicpoly-phosphonate salt includes a compound represented by ChemicalFormula 4 below:

wherein, in the t hemical Formula 4 above, n is an integer from 2 to 10.7. The cathode active material for a lithium secondary battery accordingto claim 1, wherein the lithium metal oxide particle includes anickel-based lithium oxide represented by General Formula 1 below:Li_(x)Ni_(y)M_(1-y)O₂  [General Formula 1] wherein, in the GeneralFormula 1 above, 0.95≤x≤1.08, y≥0.5, and M is at least one elementselected from a group consisting of Co, Mn, Al, Zr, Ti, B, Mg and Ba. 8.The cathode active material for a lithium secondary battery according toclaim 7, wherein, in the General Formula 1, 0.8≤y≤0.93. 9.The cathodeactive material for a lithium secondary battery according to claim 7wherein, in the General Formula 1, M includes Co and Mn.
 10. The cathodeactive material for a lithium secondary battery according to claim 9,wherein the lithium metal oxide particle includes a doping or a coatingwhich contains at least one of Al, Zr or Ti.